A typical known nitride compound semiconductor for semiconductor electronic devices is a nitride compound semiconductor having an AlGaN—GaN heterojunction. This nitride compound semiconductor unit is composed of, for example, a substrate, a nitride compound semiconductor stack on the substrate, source and drain electrodes, and a gate electrode between the source and drain electrodes. The substrate is a sapphire, Si, or similar substrate. The semiconductor stack is composed of a buffer layer, a channel layer typically made of GaN, and an AlGaN barrier layer. The source and drain electrodes make ohmic contacts with a two-dimensional electron gas formed at the interface between the barrier and channel layers.
In fabricating this known nitride compound semiconductor, it is not a very big problem if the nitride compound semiconductor stack is formed on a sapphire or SiC substrate. A Si substrate, however, has a coefficient of thermal expansion smaller than that of the nitride compound semiconductor stack. After the growth of the nitride compound semiconductor stack, the Si substrate not only becomes convex downward but can also crack because of the stress in the crystal itself.
An example of a proposed solution to this problem is methods presented in PTL 1 and 2. In these methods, two alternately grown layers with different compositions, more specifically a first layer with a composition formula of AlxInyGa1-x-yAsuPvN1-u-v (0≦x≦1, 0≦y≦1, x+y≦1, 0≦u<1, 0≦v<1, and u+v<1) and a second layer with a composition formula of AlaInbGa1-a-bAscPdN1-c-d (0≦a≦1, 0≦b≦1, a+b≦1, 0≦c<1, 0≦d<1, and c+d<1), form a multilayer buffer layer, and this multilayer buffer layer is used to mitigate the difference in the coefficient of thermal expansion between a Si substrate and a nitride compound semiconductor stack.